The present invention relates to a polyethylene type soft transparent resin sheet and producing method thereof.
Conventionally, a polyvinyl chloride sheet has been widely used as a resin sheet (or film) having flexibility, heat-resistance, and superior strength. However, the polyvinyl chloride sheet can cause bleedout of a toxic plasticizing agent or monomer during its use and can generate hydrogen chloride when it is burned, which results in environmental problems.
In order to solve the above problems, a sheet having a base of a low-stereoregular polypropylene (Japanese Patent Publication No. Hei 7-119290, Japanese Patent Laid-Open Publication No. Hei 7-171849 etc.) and a sheet having a base of an amorphous polyolefin (Japanese Patent Laid-Open Publication No. Hei 5-77371, Japanese Patent Laid-Open Publication No. Hei 6-218892 etc.) and the like have been proposed. Such sheets are ordinarily produced by a tubular film process, an extrusion process, polishing roller process and the like.
However, the sheet having the base of a low-stereoregular polypropylene does not have as much transparency, glossiness, heat-resistance and stiffness as the polyvinyl chloride sheet.
On the other hand, moldings of the sheet having a base of an amorphous polyolefin produced by either one of the tubular film process and T-die extrusion process are superior in low temperature heat sealability and brittle temperature and sometimes have mechanical characteristics such as rigidity and strength close to the polyvinyl chloride sheet. However, transparency such as gloss and haze comparable to polyvinyl chloride has not been obtained yet.
The polyethylene type soft transparent resin sheet according to a first aspect of the present invention includes layers made of a polyethylene type resin. The polyethylene type soft transparent resin sheet is characterized in having the following characteristics (a to c):
(a) Tensile modulus of 20 to 1000 MPa
When the tensile modulus is smaller than 20 MPa, the sheet is practically less valuable since the stiffness of the sheet is too weak. On the other hand, when the tensile modulus exceeds 1000 MPa, the sheet gets hard and the handling ability deteriorates, thereby making it unsuitable for the desired purpose. Preferably, the tensile modulus is 100 to 800 MPa, more preferably 100 to 600 MPa.
(b) Number of foreign substance having a refractivity different from the amorphous resin composition which occupies a major part of the volume fraction of the sheet being less than or the same as 500/mm2 at any cross section of the sheet and average length of the foreign substance being less than or the same as 10 xcexcm.
One of the major factors for impairing the transparency of the sheet is that incident light is dispersed by the foreign substance. By controlling the number of the foreign substances to be less than or the same as 500/mm2 and the average length of the foreign substance to be less than or the same as 10 xcexcm, the deterioration of the transparency of the sheet can be prevented.
The direction of the cross section is optional and any one of a perpendicular cross section and horizontal cross section relative to the front surface and back surface of the sheet can be used.
The average length refers to the mean value of the longest and shortest length of any cross section of the foreign substance, i.e. a diameter when the foreign substance is a sphere, for instance.
The body of the foreign substance is, for instance, a crystalline phase included in the same resin as the aforementioned amorphous resin composition, a resin different from the aforementioned amorphous resin phase, and an organic substance other than the resins, or inorganic substances (calcium carbonate, talc etc.).
(c) Surface roughness Ra of at least one surface being less than or the same as 0.2 xcexcm.
One of the major factors for impairing the transparency of the sheet other than the dispersion by the foreign substance is the reflection of the incident light by the surface of the sheet. In other words, when the sheet has concave and convex surfaces, the light is more likely to be reflected in relation to the incident angle of the light, thereby causing so-called irregular reflection. Further, the concave and convex surfaces on the sheet exert a large influence on quality of glossiness. When the surface roughness quantitatively expressed by the center line average roughness Ra is less than 0.2 xcexcm, the irregular reflection and deterioration in glossiness can be prevented since the concave and convex surfaces on the sheet are largely decreased. More preferably, Ra is less than 0.05 xcexcm.
The above xe2x80x9csoftxe2x80x9d word means that the Tensile modulus of the sheet is less than or the same as 1000 MPa. Preferably, the Tensile modulus is less than or the same as 800 MPa, more preferably less than or the same as 600 MPa.
Any layer structure can be adopted for the polyethylene type soft transparent resin sheet. For instance, a single-layered structure or multi-layered structure including a layer made of other types of resin can be used. When the sheet has a multi-layered structure, the content ratio of the polyethylene type resin is preferably more than or the same as 50 wt. %. The multi-layered structure can be made of, for instance, co-extrusion.
The sheet according to the present invention includes a relatively thin film.
The polyethylene type soft transparent resin sheet of the present invention is harmless to the environment since the generation of toxic gas caused in the case of vinyl chloride can be eliminated. Further, since the resin sheet is made of a soft polyethylene type resin, optical characteristics such as transparency, haze and gloss can be improved as well as mechanical strength.
The polyethylene type resin may be a copolymer of ethylene and polar group substituted ethylene.
A specific example of the copolymer of ethylene and polar group substituted ethylene is at least one selected from the group consisting of ethylene-vinyl acetate copolymer (EVA), ethylene-ethylacrylate copolymer (EEA), ethylene-acrylic acid copolymer (EAA), ethylene-methacrylic acid copolymer (EMAA), ethylene methyl acrylate copolymer (EMA), metal ion crosslinking body of ethylene-methacrylic acid copolymer (EMAA) and metal ion crosslinking body of ethylene-acrylic acid copolymer (EAA).
Selected specific resin type and number or the composition ratio thereof can be optionally chosen among the above resin types.
The polyethylene type resin may be an acrylic thermoplastic elastomer.
A second aspect of the present invention is a producing method of a polyethylene type soft transparent resin sheet using a production equipment including a cooling roller and a metal endless member abutting the cooling roller through the resin sheet, the production equipment further having an elastic member opposite to a side of the metal endless member to which the resin sheet is introduced between the cooling roller and the metal endless member to be cooled. The producing method is characterized in having the steps of: introducing the molten polyethylene type soft transparent resin sheet according to the first aspect of the present invention between the cooling roller and the metal endless member so that the resin sheet approximately simultaneously touches the metal endless member and the cooling roller; and sheet-pressing and cooling the soft transparent resin sheet while elastically deforming the elastic member.
The molten resin sheet is, for instance, a resin sheet immediately after being extruded from the die of the extruder.
Fluorine type rubber, silicone type rubber, EPDM, etc. can be used as the material of the elastic member. The thickness of the elastic member is preferably more than or the same as 3 mm for obtaining sufficient areal-pressure by elastic deformation.
The metal endless member and a surface of the roller touching the resin sheet preferably have a mirror-finished surface, which has, for instance, a surface roughness of less than or the same as 0.5 S.
Stainless steel, carbon steel, titanium alloy etc. can be used as a material of the endless member. Though the endless member is of any thickness, a thickness of more than or the same as 0.3 mm is preferable in view of strength.
Since the resin sheet is sheet-pressed and cooled accompanying elastic deformation of the elastic member, the efficiency of cooling and mirror-finished surface transferring can be improved.
Further, pressing and cooling of the resin sheet can be simultaneously conducted by introducing the resin sheet between the cooling roller and the metal endless member so that the resin sheet approximately simultaneously touches the cooling roller and the metal endless belt touching the cooling roller, thereby improving the transparency of the resin sheet. If the resin sheet first touches the metal endless member or the cooling roller, the resin sheet starts being cooled and solidifies before the mirror-finished surface is transferred to both sides of the sheet.
In the present producing method, the metal endless member may be wound around at least two rollers, and the elastic member may be formed on an outer circumference of the cooling roller composing the two rollers.
In other words, the metal endless member is a metal endless belt wound around the at least two rollers in the above arrangement.
Incidentally, a cooling roller or a roller for tension control may be provided in the metal endless belt as well as the two rollers.
In the above producing method, one metal endless member may be wound around the cooling roller to run parallel against the other metal endless member; the molten polyethylene type soft transparent resin sheet according to the present invention may be introduced between the metal endless members to approximately simultaneously touch both of the metal endless members; and the soft transparent resin sheet may be sheet-pressed and cooled while elastically deforming the elastic member.
In other words, the polyethylene type soft transparent resin sheet is sandwiched and cooled by the two metal endless members in the above arrangement.
The elastic member may be formed on an outer circumference of the roller and the metal endless member may be cylindrically formed on an outer circumference of the elastic member.
In other words, the metal endless member is formed as an external layer of the roller in the above arrangement.
The temperature of the metal endless member and the cooling roller directly touching the resin sheet is preferably from dew point to 50xc2x0 C.
When the temperature of the metal endless member and the roller for cooling the resin sheet is lower than dew point, water-drops are generated on the sheet. On the other hand, when the temperature exceeds 50xc2x0 C., good transparency cannot be obtained. Preferably, the temperature is below 30xc2x0 C.
An areal-pressure when the resin sheet is sheet-pressed accompanying elastic deformation of the elastic member is preferably from 0.1 MPa to 20.0 MPa.
When the areal-pressure is lowered below 0.1 MPa, the efficiency for transferring mirror-finished surface and cooling is deteriorated, and when the areal-pressure exceeds 20.0 MPa, tension applied to the endless belt is increased, which is not preferable in view of life span.
The elastic member preferably has a hardness of less than or the same as 95 degrees (based on JIS K6301 A).
When the hardness is larger than 95 degrees, resin banks are likely to be generated on account of weak elasticity when the resin sheet approximately simultaneously touches the cooling roller and the metal endless member. Preferably, the hardness is less than or the same as 70 degrees.
A producing method of polyethylene type soft transparent resin sheet according to third aspect of the present invention uses production equipment having a water tray provided with a slit for flowing cooling water, a water tank located beneath the water tray and a pair of sandwiching rollers disposed in the water tank with at least a portion thereof being sunk in water. The method is characterized in having the steps of: inserting a molten polyethylene type soft transparent resin sheet according to the present invention to the slit to be cooled by the cooling water; and subsequently introducing the resin sheet into the water in the water tank through a gap between the pair of sandwiching rollers.
The temperature of the cooling water is preferably less than or the same as 10xc2x0 C. Calcium chloride may be added in the water as necessary.
Annealing treatment is preferably conducted to the resin sheet obtained by the above producing method.
The annealing treatment is conducted at a temperature of 80 to 130xc2x0 C., preferably at 110 to 130xc2x0 C. The hardness of the sheet surface can be increased by the annealing treatment.